If this was a hangover from that Rafael Nadal beating in Indian Wells last Sunday, Andy Murray needs an Alka-Seltzer and a good night's sleep. The 21-year-old, whose ambition this week is to reach No3 in the world, made heavy weather of beating Juan Mónaco, an Argentine ranked 59th.

In the end, the score looks decisive enough – 4-6 6-3 6-2 – but, for the most part, this was a struggle under the Miami sun, the sort of contest for Murray, who lost his first match here last year to Mario Ancic, will want to put behind him. Perhaps the best news of the day was the defeat of David Nalbandian, the talented but unpredictable Argentine who lost in straight sets to Serbia's Victor Troicki. Nalbandian was in Murray's quarter.

Murray, who will play Chile's Nicolas Massu in the next round, admitted he had struggled in the first set. "He was hitting the ball better than me from the back court and we had some tough rallies," he said. "It took my breath away a little. I realised I needed a cap [for the sun] – I was having to slow down my serve from one end of the court. Once I started hitting my serve harder and winning some free points, the match changed. It wasn't my best performance but the only thing that matters is winning."

Murray began as if he had sand in his shoes and fuzz in his brain. He was frequently late as his backhand failed to cope with the Argentine's power-packed forehand drives and his decisions as to when to get to the net seemed just too late to deal with the return.

He soon realised he should have brought a cap on court and began fretting about it with signals to his corner where, this week, the bevy of coaches and trainers has been augmented by mother Judy. For the first hour she must have wondered whether she would have been better off at home.

When he put a routine backhand wide, Murray handed Mónaco the first break and worse was to follow when he chipped and charged on break point down and got passed down the line. That made it 1-4 and alarm bells were ringing – much to the glee of the crowd which is always biased towards South American players out here on the Florida Keys. There were signs of the world No4 re-emerging as Murray broke back in the sixth game but he was soon all over the place again, having to dig himself out of three break points in the next game.

But the most worrying aspect was encapsulated in the next point when another 20-plus stroke rally developed and it was Murray who cracked by netting a forehand stroke.

The second set was fraught. Murray began by having to dig himself out of 0-40 and then missed a break point in the next game when Mónaco pulled off one of those gets that suggest a player has all the stars aligned in his favour. Coming in to deal with a short ball, Murray hit it perfectly – a low cross-court screamer that had winner written all over hit. But the Argentine ran, lunged and somehow lifted it back over his opponent's head for a perfect winner.

But the Scot stuck to his task and Mónaco's backhand started to cave in, and when a smash went wide it cost him the vital break in the fourth game. Murray managed to serve out for parity despite needing to save a break point while doing so and then took charge of the match after serves had been swapped at the top of the third.